Depends on the vehicle in my opinion.
I wouldn't put it on "old" paint. For sure not unless it has a clear coat, and even then I would hesitate depending on age of paint.
My 2013 Charger I have never waxed or anything in ten years. I run it through auto washes. It is black, all of it. I used a waterless wash product last fall(will soon this spring again) and it looked like new. Water beads and rolls off. I have never applied a product to it, ever.
Course, it also spends life at home in the climate controlled garage. it does sit outside at work though, in WI rain, shine, snow, sleet, salt.
This car, if I was to pick a product to put on, would probably get the ceramic. Prep does matter with that stuff, but I think factory paint type matters more.
90's era clear coat paint jobs I would stick with wax.
Non-clear paint gets ONLY carnauba. But those types of vehicles are not usually commuters, or not the type to worry about(trucks)